My guess here, because of the way you got it to work, is that in the first code you are using the global variable $post somewhere that it is not defined.
Welcome
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
We use cookies to offer you a better browsing experience, analyze site traffic and personalize content. Read about how we use cookies and how you can control them in our Cookie Policy. If you continue to use this site, you consent to our use of cookies.